1. Field of the Invention
The present invention relates to wireless communications systems and signal processing apparatus employed in wireless communications systems. More particularly, the present invention relates to communications systems that transmit simultaneously on multiple carriers.
2. Background of the Prior Art and Related Information
Wireless communications systems employing transmission between base stations and remote users are a key component of the modern communications infrastructure. These wireless systems are being placed under increasing performance demands which are taxing the capability of available equipment, especially wireless base station equipment. These increasing performance demands are due to both the increasing numbers of users within a given wireless region, as well as the bandwidth requirements allocated to wireless system service providers. The increasing number of wireless users is of course readily apparent and this trend is unlikely to slow due to the convenience of wireless services. The second consideration is largely due to the increased types of functionality provided by wireless systems, such as wireless Internet access and other forms of wireless data transfer over such systems. These considerations have resulted in a need for more carriers operating from each transmitting location of a wireless service network.
When transmitting multiple carriers from a single location, it is advantageous to combine carriers as early in the signal generation process as possible. By doing so, the transmitting location can reduce the number of antennas, low loss cables and power amplifiers. The ultimate goal is to combine individual channels with digital signal processing before RF signal generation. In some multiple carrier systems like OFDM (orthogonal frequency division multiple access), it is best to combine carriers with digital signal processing in order to maintain the required orthogonal carrier characteristics.
An example of a prior art multiple carrier signal generator is shown in FIG. 1. Information symbol streams are input from the individual carrier data handling systems. These symbols are generally complex valued data points that represent one or more information data bits to be transmitted. The symbol streams are then upsampled to a higher sample rate by inserting equally time spaced zero valued samples between the input symbols. The upsampled symbol streams are then passed through baseband filters to create the individual carrier baseband signal waveforms. After the individual carrier baseband signals are created, each carrier is offset in frequency and combined producing a multiple carrier communication signal. This signal is then digital-to-analog converted and up-converted in frequency to the desired operating bandwidth.
A problem exists, however, with the above multiple carrier generation process. Individual carriers in the above process obey power statistics that are a function of the symbol generation process used and the baseband filter impulse response function characteristics. These individual carrier power statistics are generally described by the carrier power complementary cumulative probability density function (CCDF), and from that function, the signal power peak-to-average ratio. Signals with high peak-to-average ratios create problems in digital-to-analog conversion, RF signal generation, and RF power amplification. Even if the peak-to-average power ratios of individual carriers is low, or can be maintained below a maximum level using signal-processing algorithms, combining these multiple carriers may produce high peak-to-average ratios. These high multi-carrier peak-to-average ratios once again cause problems in digital-to-analog conversion, RF signal generation, and RF power amplification.
Signals with high peak-to-average ratios cause the following problems in communications systems. First, the number of significant digits used to calculate the signal must be large enough to maintain adequate signal resolution when the signal is both very large and very small. Second, at the output of the digital signal processor are digital-to-analog converters. To accommodate a digital signal with a high peak-to-average ratio, high bit count digital-to-analog converters must be used so that both large and small values can be generated. If this is not done the output signal will have a poor output signal-to-noise ratio. Finally, signals with high peak-to-average ratios require very linear analog, IF, RF and RF power amplifier circuits. Without these very linear circuits, distortion products are generated at frequencies outside the government allocated bandwidth of the wireless system license. Such highly linear circuits are expensive, however, and add considerable cost to the system.
Accordingly, a problem presently exists in multiple carrier communication systems due to large peak-to-average power ratios occurring after carrier combination.
The present invention provides a system and method for signal peak reduction in a multiple carrier communication system where the individual carriers are produced from input upsampled symbols that are filtered to reduce individual carrier bandwidths, offset in frequency, and combined into an output signal.
In a first aspect, the present invention provides a multi-carrier communication system. The multi-carrier communication system comprises a plurality of separate carrier signal sources, each providing symbols corresponding to one or more data channels. A peak reduction stage is coupled to receive the plurality of separate carrier symbols and outputs peak reduced symbols for each carrier based on the effect of the other carriers. A frequency offset stage is provided for shifting the frequency of each carrier signal and a combining stage combines the frequency shifted carrier signals into a multi-carrier signal. An RF up conversion stage may be configured before or after the combining stage. Also, a digital-to-analog conversion stage for converting carrier symbols to analog signals may be configured before or after the combining stage.
The peak reduction stage preferably comprises a phase shift circuit providing a separate frequency offset output for each carrier, a summing circuit summing the frequency offset outputs, an algorithm processor receiving the output of the summing circuit and calculating symbol adjustment values based on the output of the summing circuit and a peak power limit value, and a combining circuit combining symbol adjustment values and input carrier symbols. The multi-carrier communication system of the present invention, may further comprise a plurality of filters equal to the number of separate carriers coupled to the output of the peak reduction stage. In such an embodiment, the peak reduction stage preferably further comprises a filter predictor for each filter, wherein the filter predictor outputs are provided to the phase shift circuit.
In a further aspect the present, invention provides a multi-carrier communication system, comprising a plurality of separate carrier signal sources, each providing a stream of carrier symbols corresponding to one or more data channels A plurality of filters equal to the number of separate carriers each provides a filtering operation based on a filter impulse response function. A peak reduction unit is coupled between the plurality of carrier signal sources and the plurality of filters and receives the carrier symbols from the plural carrier signal sources. The peak reduction unit includes a plurality of filter predictors which provide predicted filtered outputs for each of the plurality of filters using filter coefficient values corresponding to samples of the filter impulse response function of each of the filters. The peak reduction unit further includes a peak reduction algorithm circuit block for receiving the predicted filtered outputs and determining peak reduction values for each stream of carrier symbols, and a plurality of combiners combining the peak reduction values and carrier symbols and providing peak adjusted carrier symbols. The peak reduction unit may further comprise a plurality of delay circuits for delaying the carrier symbols so that the plurality of combiners receive the peak reduction values and the carrier symbols on a symbol-by-symbol basis in a time synchronized manner. The multi-carrier communication system further comprises a plurality of frequency offset circuits equal in number to the plurality of separate carrier signals and a carrier combiner for combining the outputs from the plural frequency offset circuits to provide a multi-carrier output.
The multi-carrier communication system may further comprise a digital-to-analog converter for converting the multi-carrier output to a multi-carrier analog signal and an RF mixer for mixing the multi-carrier analog signal with an RF carrier and providing a multi-carrier RF output. A plurality of up sampling circuits, coupled between the peak reduction unit and each of the filters, may also be provided for increasing the sampling rate of the peak adjusted carrier symbols prior to filtering. The up sampled symbols are then filtered at the up sampled filter rate.
A number of other features and different embodiments may also be provided. For example, at least some of the filters may employ different impulse response functions and the filter predictors receive filter coefficients corresponding to the different filter impulse response functions. The peak reduction unit may comprise a plurality of weighting circuits for weighting the peak reduction values based on the instantaneous power of each carrier. Each of the combiners may comprise a multiplier circuit and the peak reduction value comprises a gain which when multiplied by the carrier symbol provides an adjusted carrier symbol. Alternatively, each of the combiners may comprise an addition circuit and the peak reduction value comprises a value which when added to the carrier symbol provides an adjusted carrier symbol. The peak reduction algorithm circuit block may comprise a phase shift circuit for phase shifting each filter predictor output and an algorithm processor for calculating peak reduction values. The peak reduction algorithm circuit block may further comprise a summer for summing the phase shifted filter predictor outputs and providing an output to said algorithm processor. The peak reduction algorithm circuit block may further comprise a magnitude detection circuit for detecting the magnitude of the summer output and a comparator for comparing the magnitude of the summer output to a peak limit value. The peak reduction algorithm circuit block may further comprise a selector switch coupled to the comparator and enabling the peak reduction value to be output to a combiner when said switch is enabled by the comparator. The peak reduction unit may further comprise a plurality of feedback loops which provide the peak reduction values to the filter predictors. The filter predictors may each include a memory register comprising a plurality of delay stages and the feedback loops provide said peak reduction values to said memory registers between the delay stages. The peak reduced carrier symbols may be output from the memory registers.
In yet another aspect, the present invention provides a multi-carrier communication system, comprising a plurality of separate carrier signal sources, each providing a stream of carrier symbols corresponding to one or more data channels and a plurality of up sampling circuits, corresponding to the number of separate carrier signal sources, for increasing the sampling rate of symbols input thereto and providing up sampled symbols. The multi-carrier communication system further comprises a plurality of filters for providing filtering operations based on one or more filter impulse response functions and employing filter coefficients corresponding to a timing based on the increased sampling rate. A peak reduction unit is coupled between the plurality of separate carrier signal sources and the filters and receives the carrier symbols from the signal sources and provides peak adjusted carrier symbols. The peak reduction unit includes a plurality of peak reduction stages, each stage predicting the effect of the filters on the data symbols using filter coefficient values corresponding to a portion of the total number of sample points of the filter impulse response function to provide predicted filtered outputs and providing a peak reduction processing based on predicted filter outputs. The peak reduction unit provides the peak adjusted carrier symbols, after the plural stage peak reduction processing, to the filters. The multi-carrier communication system further comprises a plurality of frequency offset circuits, equal in number to the number of separate carrier signal sources, for forming frequency offset carrier signals corresponding to the peak adjusted carrier symbols, and a combiner for combining the frequency offset carrier signals to form a multi-carrier signal.
A number of various features and different embodiments may also be provided. For example, the plural stages of the peak reduction unit may be provided in series. Alternatively, the plural stages of the peak reduction unit may be provided in parallel. Also, each stage of the peak reduction unit may comprise a plurality of filter predictors receiving filter coefficients corresponding to a portion of the total number of sample points of the filter impulse response function and providing said predicted filtered outputs, and a peak reduction algorithm circuit block for calculating a peak reduction value based on the predicted filtered outputs provided by the filter predictors. Each stage of the peak reduction unit may thus apply N filter coefficients to the filter predictors corresponding to N sample points of the impulse response function.
In yet another aspect, the present invention provides a method for signal peak reduction in a multiple carrier communication system where the individual carriers are produced from input symbols that are filtered to reduce individual carrier bandwidths, offset in frequency, and combined into an output signal. The method comprises predicting the effect of filtering on input symbols for each carrier and providing predicted filtered symbols for each carrier. The predicted filtered symbols are phase shifted by a separate amount for each carrier and combined. Peak reduction adjustment values for each carrier are determined based on the amount the combined phase shifted predicted filtered symbols exceed a threshold peak power value. The input symbols are adjusted using the peak reduction adjustment values.
A number of various features and different embodiments may also be provided for this aspect of the invention. For example, the act of adjusting the input symbols may comprise adding peak reduction adjustment values to input symbols for each carrier. Alternatively, the peak reduction adjustment values may comprise peak reduction adjustment gain values and the act of adjusting the input symbols may comprise multiplying input symbols for each carrier by the peak reduction adjustment gain values. The act of determining peak reduction adjustment values may comprise determining a single peak reduction adjustment value and phase shifting the peak reduction adjustment value by a separate amount for each carrier to create plural peak reduction adjustment values. The act of determining peak reduction adjustment values may further comprise determining a weighting value for each carrier and weighting the plural peak reduction adjustment values by corresponding weighting values. The act of determining a weighting value for each carrier may comprise determining the instantaneous power of each carrier and the act of weighting the plural peak reduction adjustment values by corresponding weighting values may comprise multiplying the peak reduction adjustment values of each carrier by the instantaneous power of the carrier. The method for signal peak reduction in a multiple carrier communication system may further comprise repeating the acts of predicting, phase shifting, combining, determining and adjusting a plurality of times employing different filter coefficients. The plural repetitions of the acts of predicting, phase shifting, combining, determining and adjusting may be performed in series. Alternatively, the plural repetitions of the acts of predicting, phase shifting, combining, determining and adjusting may be performed in parallel.
The present invention thus provides peak power reduction of multi-carrier communication systems. By doing so, the complexity and cost of the digital-to-analog converters, analog, IF, FR and RF power amplifier circuits in such communication systems will be greatly reduced. Other features and advantages of the present invention will be appreciated by a review of the following detailed description of the invention.